In semiconductor manufacture, extremely small electronic devices are formed in separate dies in a thin, flat semiconductor wafer. In general, various materials which are either conductive, insulating, or semiconducting are utilized in the fabrication of integrated circuitry on semiconductor wafers. These materials are patterned, doped with impurities, or deposited in layers by various processes to form integrated circuits.
Increasing circuitry miniaturization and a corresponding increase in density has resulted in a high degree of varying topography being created on an outer wafer surface during fabrication. It is often necessary to polish a wafer surface having varying topography to provide a substantially planar surface. One such process is chemical-mechanical polishing. In general, this process involves holding and rotating a thin, flat wafer of the semiconductor material against a wetted polishing surface under controlled chemical, pressure, and temperature conditions. A chemical slurry containing a polishing agent, such as alumina or silica, is utilized as the abrasive medium. Additionally, the chemical slurry contains selected chemicals which etch various surfaces of the wafer during processing. The polishing effect on the wafer results in a chemical and mechanical action.
A particular problem encountered in chemical-mechanical polishing is the determination that the surface has been planarized to a desired end point. It is often desirable, for example, to remove a thickness of oxide material which has been deposited onto a substrate, and on which a variety of integrated circuit devices have been formed. In removing or planarizing this oxide, it is desirable to remove the oxide to the top of the various integrated circuits devices without removing any portion of the devices. Typically, this planarization process is accomplished by control of the rotational speed, downward pressure, chemical slurry, and time of polishing.
The planar endpoint of a planarized surface is typically determined by mechanically removing the semiconductor wafer from the planarization apparatus and physically measuring the semiconductor wafer by techniques which ascertain dimensional and planar characteristics. If the semiconductor wafer does not meet specification, it must be loaded back into the planarization apparatus and planarized again. Alternately, an excess of material may have been removed from the semiconductor wafer, rendering the part as substandard.
Certain techniques have also been developed for in situ detection of chemical-mechanical planarization. Typically these techniques rely on measurements of the physical thickness of the layer being polished, or judge end point from electrical changes that occur when the polishing layer is completely removed. Such are disclosed, by way of example, in U.S. Pat. Nos. 4,793,895; 5,036,015; 5,069,002; 5,081,421; and 5,081,796.
A further issue in chemical-mechanical planarizing in some cases is achieving a desired planarity and removing a minimum amount of the material being planarized. For example in a process optimized for throughput, the amount of removed material is adjusted to be the minimum amount necessary to achieve a desired result. In a planarizing process, the desired result is to have a completely planarized end surface.
It would be desirable to develop improved methods of chemical-mechanical polishing, and improved methods of end point detection in chemical-mechanical polishing.